Auto Electronics

Power Electronics Challenges for Hybrids

Suppliers have been busy improving existing designs and inventing new technologies to make hybrids and PHevs more acceptable for vehicle buyers.

Before the Escalating Oil Prices of 2008, Nomura Research Institute forecasted a global hybrid market of 2.2 million units per year by 2012 with sales in 2007 of 619,000 units[1]. These numbers were largely driven by Toyota volumes. As more automakers enter the hybrid vehicle market and increase the variety of hybrid vehicles offered, including plug-in hybrid electric vehicles (PHEVs), it will be interesting to see the predictions for these vehicles at the end of 2008. At the same time, more suppliers have been challenged to address the problems identified with initial hybrid products.

Several advancements have occurred for improved performance and reliability as well as much needed cost reduction. In addition to the well-known thrust for improved batteries, power electronics provides an area where increased volume and competition will continue to fuel a virtuous circle for hybrid acceptance and growth.

IGBTS AND PACKAGING

At the Applied Power Electronics Conference (APEC) 2008 in Austin Texas, Steven Schultz, a technical specialist in controls engineering for General Motors noted that power electronics accounts for 20% or more of the cost of a hybrid vehicle's material costs — almost equaling the cost of the battery. Starting at the semiconductor level, the main power component is the insulated-gate bipolar transistor (IGBT) used in the motor control inverter. While IGBT wafer demand for electric vehicles dwarfs industrial motor drive usage and is even small compared to home appliances today, by 2012 vehicle requirements will greatly exceed home appliances and almost match industrial drives according to the IMS2005 data that Schultz presented. If some suppliers have been slow to respond to automakers' specific needs, this increase in volume could improve their interest. With its 2-mode hybrid system, Chevy Volt extended-range EV and other hybrid designs, GM certainly is looking to suppliers for product improvements and cost reduction.

Existing IGBT technology was developed for 600 V and 1200 V industrial and appliance applications. “In industrial, you have two line voltages that dominate. One is 230 V and the other is 480 V,” said Tony O'Gorman, Distinguished Member of the Technical Staff, Continental Automotive. “When you have a 230 V line and you rectify it, a 600 V device works fine.”

In contrast, the automotive voltage range on hybrids provides unique challenges. “In some cases, an intermediate voltage such as 900 V could provide a better solution,” said GM's Schultz.

At the same time, automotive battery voltages for hybrids have been increasing. One automotive semiconductor supplier has responded to this challenge. “We are making a new class of devices specifically for hybrids, plug-in and EV applications,” said Sayeed Ahmed, senior manager, Regional Marketing North America, Infineon Technologies. “With battery voltages creeping up from 350 V to 400 V to 450 V, 600 V breakdown was at the borderline.” As a result, Infineon is changing the voltage class for the silicon to 650 V. To meet the unique automotive cycling requirements, the top metal on these semiconductor devices is being increased as well.

Improved protection and control can dictate the need for the integration of additional features on an IGBT. From the silicon design perspective, on-chip current sen-sing and temperature sensing are possible but Ahmed indicated that only the current sensing capability is being explored with automotive customers. The cost increase associated with the temperature sen-sing has not made it attractive.

One auto company that has taken control of its hybrid destiny is Toyota. In his presentation, “Evolution of Hybrid Vehicle Electric System and its Support Technologies” at APEC 2007 in Anaheim, CA, Kimimori Hamada of Toyota Motor Corporation provided an interesting summary of the improvements to IGBTs since the introduction of the first Prius Hybrid. Figure 1 shows three different levels of IGBT design. The most recent change came from an electric field dispersion (EFD) technique that reduced on-state losses while also allowing thinner wafers. This provided increased performance with reduced cost since the wafers used less silicon.

Toyota increased the breakdown voltage from 970 V for the Prius to 1200 V for the Lexus RX400h SUV to handle the higher torque and output power that the THSII system required. At the same time, the die size was reduced and on-state losses were reduced, too. Figure 2 shows a summary of the silicon changes over three different vehicle models.

Semiconductor technology is only one aspect of the power electronics. “Automotive is a very harsh environment, yet we expect very high reliability, so it places a big burden on the power packages to not only keep the junction cool but also to prevent failure after many, many thermal cycles,” stressed GM's Schultz.

For a semiconductor supplier to prove its product meets the automotive requirement, additional testing is required. “To qualify for the automotive market you need to do more testing and the power cycling capability of the devices and packaging needs to be improved,” said Infineon's Ahmed. In addition to changes that Infineon made at the silicon level, they also implemented unique packaging changes.

To provide a module with 800A capability, instead of wirebonding from the substrate to the terminals, Infineon implemented ultrasonic terminal bonding for both power and signal connections that is shown in Figure 3. The thicker copper terminal reduces the heat generated into the substrate and avoids attaching as many as 40 aluminum wirebonds to the baseplate. Moving the wirebonding head also requires a lot of space. So the ultrasonic approach used for both power and auxiliary connectors provides improved manufacturability.

“For the future, we are raising the junction temperature of the devices to a maximum up to 200°C,” said Ahmed. The existing silicon can already handle this temperature, so packaging changes are the enabler for the higher temperature capability. Improvements in wirebond material and die attach solder process materials are among the changes being made to withstand these higher temperatures.

Working directly with Honda, another semiconductor supplier recently announced what could be a breakthrough for silicon carbide (SiC) semiconductors in automotive hybrids. Using ROHM SiC MOSFETs and Schottky-Barrier Diodes and Honda's high power packaging expertise, the partners developed a 1,200 V/ 230 A (280 kVA equivalent) power inverter module. Figure 4 shows the module and its SiC components.

While no specific vehicle implementation has been identified, Honda and ROHM are confident that the newly announced silicon carbide power inverter module that the two companies jointly developed outperforms silicon versions. Figure 5 shows the improvements that resulted from using SiC instead of silicon for the power semiconductor devices. In addition to lower switching losses, the PWM frequency can be increased to 80 kHz for the SiC MOSFETS and Schottky diodes compared to 20 kHz for the silicon IGBT version. The higher frequency can reduce the size and cost of passive components.

AUTOMOTIVE SAFETY

With the high voltages used in hybrids, automotive safety takes on new meaning. “One of the things that the hybrid vehicle brings to the auto industry is safety concerns,” said Dennis Stephens, principal staff engineer, Continental. “We have to think about the fact that now we have 300, 400, 500 volts floating around in a car, so there are a lot of redundant systems in place, both hardware, software, and interlocks that prevent the user from ever getting to those voltages.” Ensuring safety has several different aspects.

One of the initial safety concerns is the lithium-ion chemistry. “For the old nickel-metal hydride batteries we can look at stack of cells and sort of say are we balancing,” said Stephens. “For the new lithium batteries, we have to look at every cell. So if every cell is 3 V and you have to go up to 300, you have a lot of them in series that you have to constantly monitor.”

Constantly monitoring galvanic isolation is another safety-critical area. This requires sensing that the battery is isolated from the chassis. “There is some microprocessor brain power inside these batteries and battery management systems to constantly monitor that,” said Stephens. Contactors inside the batteries open up and turn off the battery if any fault protection flag goes high. This interlock capability prevents users from accessing the high-voltage terminals.

PHEVS have another problem that is different from existing hybrids. “Right now, a vehicle floats above the ground,” says Stephens. “When my car is plugged into the outlet in the garage, now I have mains voltage on my car.” The issues that this creates have to be worked out. Perhaps the bigger issue is the standards that need to be established that “aren't there yet” according to Stephens.

MODEL-BASED DESIGN FOR HYBRIDS

Model-based design (MDB) may provide answers to new components, safety and the need for overall development of a vehicle that is even more complex than one with just an internal combustion engine.

“For hybrid vehicles, because you are putting a lot of new components together, you really have to optimize your design on the top level, on the system level, and this is where modeling, simulation and model-based design can help engineers, ” said Wensi Jin, Automotive Industry marketing manager, The MathWorks. A tool recently introduced by The MathWorks, called Simscape language specifically simplifies the effort of engineers to implement newly developed components into existing models. The new IGBTs mentioned earlier provide a good example.

“If they are optimizing one component, they can take out the original block from a shipping product for an IGBT, describe their own IGBT in Simscape language and essentially this new block will go into the system level models,” said Jin. As shown in Figure 6, Simscape allows engineers to describe a component in language and generate a graphical model. Simscape is based on the widely used MATLAB language. The difference is instead of a data-flow-oriented programming environment, Simscape uses a network approach, an acausal modeling environment for modeling the physical network.

With model-based design and Simscape, designers can model on different levels and focus on those areas that specifically impact safety. In an accident situation, safety involves the disconnecting of high-voltage components. “Things that are very difficult to do in the real vehicle, you can do easily in the laboratory environment with models,” said Jin. “You can model the fault management system and be able to run these models in a modeling system environment.”

In addition to its use in the model-based design, The MathWorks Real-Time Workshop Embedded Coder product for MathWorks Release 2008a was recently certified by TÜV SÜD Automotive GmbH. The certification was for safety-related development according to IEC 61508. This could be a step toward a basis for standardization of at least one aspect of the high-voltage system. TÜV granted the certificate based upon a workflow for typical automotive applications that addresses “Application-Specific Verification and Validation of Models and Generated Code.”

LITHIUM-ION BATTERIES CHARGE TOWARD PRODUCTION VEHICLES

Software optimized batteries are one thing but producing automotive-grade lithium-ion batteries is another. Improved batteries have been the weak link in hybrid and electric vehicles since the first EV hit the road in the early 20th century, but the final critical piece of the hybrid, PHEV and EV could be falling into place. Near the end of the third quarter of 2008, Continental AG opened a factory in Germany to manufacture lithium-ion batteries for hybrid vehicles. The battery will initially be used in Mercedes-Benz S 400 BlueHYBRID that will be in production in 2009. As shown in Figure 7, the battery is compact enough to mount under the hood of the vehicle.

Continental's battery has a volume of 13 liters, weighs 25 kilograms and allows electric motors in cars to supplement the output of combustion engines by up to 19 kilowatts of power. The battery system consists of the lithium-ion cells and the cell monitoring system, the battery management function, high-strength housing, cooling gel, a cooling plate, a coolant feed and the high-voltage connectors. Integrated electronic circuitry monitors the battery's overall health, temperature and energy capability as the system ages. If excessive temperatures occur, a safety interlock switches the battery off. A Cell Supervision Circuit (CSC) monitors the status and controls the interaction of the single cells. The CSC adjusts the charge condition to ensure all cells are loaded equally.


References

  1. http://www.nri.co.jp/english/opinion/papers/2007/pdf/np2007114.pdf.


ABOUT THE AUTHOR

Randy Frank is president of Randy Frank & Associates Ltd., a technical marketing consulting firm based in Scottsdale, AZ. He is an SAE and IEEE Fellow and has been involved in automotive electronics for more than 25 years. He can be reached at [email protected].

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